The invention relates to fixing toner images in electrophotograpy, and is specifically concerned with methods and devices relating to fixing an image formed by toner particles by heating and drying the image.
In dry type electrophotographic processes, a toner image which is on a paper sheet or on another supporter is fixed by heating the sheet. For example, the sheet may be heated by being brought in direct contact with a heated surface, such as, for example, the peripheral surface of a heating drum. One disadvantage of contact heating is that the toner particles forming the image are rendered tacky by heating, and portions of the toner image may adhere to the periphery of the heating drum. As a result, when the next sheet bearing a toner image is brought into contact with the heating drum, the tacky toner particles removed from the preceding sheet may be transferred to that next sheet. At the same time, portions of the toner image supported on that next sheet may adhere to the periphery of the heating roller, and the cycle may be repeated. The phenonmenon of portions of the toner image of one sheet adhering to the heating drum and being transferred to the next sheet is generally referred to as the "ofset phenomenon", and it is detrimental to the quality of the fixed image.
In an effort to preclude or reduce this offset phenomenon, heating drums have been covered with layers of a highly non-tacky material, such as silicon rubber or Teflon. Different results are achieved with different materials. Generally, materials having a higher percentage of organic components are preferred. Materials suitable for this purpose are rubber compounds made of raw silicon rubber containing dimethylpolysiloxane as a principal raw material and vulcanized at normal or low temperature. However, raw silicon rubber is expensive, and the cost of producing a cover layer of this type is further increased because various fillers, extending agents, and vulcanizing agents are also added to the raw material.
Even if such expensive cover layers for heater drums are used, it may be impossible to completely preclude the offset phenomenon. In fact, it has not been ascertained yet what is the optimal type of a heating drum surface which would completely preclude the offset phenomenon. All that has been known up to now is that the surface condition of the heating drum has an important bearing on the adhesion of toner particles to the next sheet bearing a toner image; it has not been known how best to prevent this offset phenomenon.
The fact is that the offset phenomenon has remained a problem in fixing toner images, and it is therefore desirable to find ways to completely or substantially preclude the offset phenomenon.